This invention relates generally to implantable medical stimulators, and more particularly to implantable pacemaker/cardioverter/defibrillators.
Implantable pacemaker/cardioverter/defibrillators, of the types presently in clinical evaluation include the capability to detect the occurrence of tachycardias and fibrillation. In response to detection of tachycardias, anti-tachycardia pacing therapy or high voltage cardioversion pulses may be delivered. In response to detection of fibrillation, high voltage defibrillation pulses are typically the only therapy delivered. Traditionally, anti-tachycardia pacing and cardioversion have been used as alternative therapies, with cardioversion typically used when anti-tachycardia pacing fails to terminate a detected tachycardia, when a detected tachycardia accelerates or when the rate of the detected tachycardia meets a predetermined fast tachycardia rate threshold. U.S. Pat. No. 4,830,006, issued to Haluska et al., U.S. Pat. No. 4,971,058, issued to Pless et al., U.S. Pat. No. 5,161,527, issued to Nappholz et al. and U.S. Pat. No. 5,002,052, issued to Haluska disclose devices which function in these fashions.
High voltage cardioversion and defibrillation pulses are typically delivered using high voltage output capacitors not used for delivery of pacing pulses. These capacitors are typically charged in response to detection of a tachycardia or fibrillation, rather than maintained in a charged condition. As a result, a substantial amount of time may pass between detection of the arrhythmia and delivery of the high voltage pulse.
High voltage pulses are extremely painful in many cases and impose a substantial current drain on the battery of the implanted device. Therefore, avoiding inappropriate delivery of such pulses is recognized to be desirable. It has been recognized that the arrhythmia may change during charging. As a result, sensing the rhythm during charging has been proposed. For example in U.S. Pat. No. 4,949,719, issued to Pless et al, and U.S. Pat. No. 5,191,884, issued to Gilli et al, the implanted device checks during charging to determine whether the arrhythmia has spontaneously terminated and aborts the charging of the output capacitors if the rhythm has returned to normal. While this feature in some cases accomplishes the desired result of avoiding unnecessary shocks, further reductions in the frequency of delivery of such shocks are still desirable.